In the manufacture of cellular products from foamed polymers the size of the cells has significant effects on many of the physical properties of such products. For example, thermal insulating and compressive strength properties are directly related to cell size. To establish manufacturing procedures for the production of cellular products having the desired physical characteristics, therefore, it is important to be able to determine accurately the size of cells resulting from various production techniques. Once the cell size resulting from one production technique has been established, the technique may be maintained or varied, as required, to produce a cellular product corresponding to predetermined specifications. To ensure that the appropriate technique is being maintained, it is desirable to be able to monitor the production by making rapid examinations of samples of the product at frequent intervals during its production run.
The importance of cell size to the physical properties of a particular product is well known and at least two proposals have been made heretofore for determining cell size. Each proposal involves cutting a thin section from a foam sample and then, according to one proposal, transmitting light through a predetermined area of the sample to produce an image and then projecting the image for visual inspection. In the second proposal a predetermined area of the surface of a thin sample is illuminated, and light reflected by such surface of the sample is transmitted to a microscope for visual examination.
Both proposals rely on the ability of a viewer to distinguish the contrast between the intensities of the light transmitted through or reflected by the sample at the walls of the cells and at the exposed cavities created by cutting of the sample. In many instances the intensities of the transmitted or reflected light are too nearly equal to enable the observer to discriminate accurately between the cell walls and the cell cavities, thereby resulting in significant errors in determining cell size. Theoretically, the problem in distinguishing between cells and cell walls may be overcome by forming a sample having a thickness corresponding substantially to the diameter of a single cell. In practice, however, it is not possible to form such samples consistently. As a consequence cell walls or struts internal of the sample cannot be distinguished from those walls or struts at the surface of the sample. The inability of an observer to distinguish between cell walls at and below the surface of a sample is particularly pronounced in those instances in which the sample is formed of material which is transparent. Thus, it is virtually impossible to obtain accurate cell size determination using currently available methods.
A distinct disadvantage resulting from the slicing of a thin section sample from a body of foam material is that the slicing equipment presently available invariably causes the formation of irregularities in the exposed surfaces of the cell walls, thereby preventing the preparation of a truly representative sample. For example, many polymers are so fragile that the slicing operation results in the tearing away of sections of a wall and the formation of ragged edges. Particularly is this true in those instances in which the sample has a thickness approaching that of only one cell diameter.
A further disadvantage of the known methods of examining samples of the kind referred to is that the inspection requires too great a time to complete. Thus, if an inspection reveals deficiencies in a product, it is possible that a substantial quantity of the deficient product will have been produced during the time it takes to complete the inspection and before corrective adjustments can be made in the production process.
Apparatus and methods according to the invention overcome the disadvantages referred to above and are applicable not only to the examination of opaque, translucent, and transparent cellular products, but to other objects as well.